.. actually, by a second look at current doc + quick look at famsg source code, i kinda understand it does similar file lookup as done by include directive - except the %include% env variable part... Am I correct ?
Yes, in both fasm and fasmg the FILE lookup should be the same as INCLUDE except for the environment variable paths.
Also keep in mind that your mention of %include% could be misunderstood by fasm users, because it looks like fasm's legacy feature that allows to put value of any environment variable into path, with % characters on both sides. You can see something like:
include "%fasminc%/win32ax.inc"
in very old sources. This was largely superseded by the use of multi-path INCLUDE variable, and for fasmg I decided to not implement it, because the new engine allows to process the path during assembly (with -I switch to pass values from command line, if needed).